Study of effect of pulsing sequence of nanosecond pulsed electric fields on viability of HeLa S3 cell

Here, we demonstrates the influence of the pulsing sequence of repetitive nanosecond pulsed electric fields on the viability of human cancer cells. A commercially available repetitive magnetic pulse compression generator was used to deliver 70 ns-long high voltage pulses to cultured HeLa cells in a 4 mm-gap cuvette electrode. Two kinds of pulsing sequential patterns, simply repetitive ns pulses and repetitive burst pulses, were used. The repetition frequency is varied from 0.01 to 250 pulses per second (pps), which corresponds to the interval from 100 to 0.004 s, respectively. The electric field and the number of pulses were fixed at 25 kV/cm and 25, respectively, so that the cells were subjected to the sub-lethal condition. Propidium iodide (PI), which fluoresces in red only when being intercalated to DNA, was used to identify dead cells in a population. Statistical analysis of the death ratio in thousands of cells was done with a flow cytometer. The experiment using the simply repetitive pulses shows the cell viability was changed with the repetition frequency even though the electrical dissipated energy in the medium was the same for all conditions. We found that the death ratio was increased with decreasing the pulse repetition frequency down to 0.33 pps, whereas the ratio is decreased with decreasing the frequency below 0.1 pps. Generally speaking, biological processes advance in their own time constant, for instance, the resealing mechanism of nanopores caused by the application of ns pulses lasts for minutes. The dependence of the cell viability on the pulsing sequence might be associated with time constants of biological processes related to the recovery from the field induced damage.